HERACLIDES OF PONTUS.

All authorities agree that the theory of the daily rotation of the earth about its own axis was put forward by Heraclides of Pontus (about 388–315 B.C.), a pupil of Plato; with him in some accounts is associated the name of one Ecphantus, a Pythagorean. We are told that Ecphantus asserted “that the earth, being in the centre of the universe, moves about its own centre in an eastward direction,” and that “Heraclides of Pontus and Ecphantus the Pythagorean make the earth move, not in the sense of translation, but by way of turning as on an axle, like a wheel, from west to east, about its own centre”.

Heraclides was born at Heraclea in Pontus. He went to Athens not later than 364 B.C., and there met Speusippus, who introduced him into the school of Plato. On the death of Speusippus (then at the head of the school) in 338, Xenocrates was elected to succeed him; at this election Heraclides was also a candidate and was only defeated by a few votes. He was the author of dialogues, brilliant and original, on all sorts of subjects, which were much read and imitated at Rome, e.g. by Varro and Cicero. Two of them “On Nature” and “On the Heavens” may have dealt with astronomy.

In his view that the earth rotates about its own axis Heraclides is associated with Aristarchus of Samos; thus Simplicius says: “There have been some, like Heraclides of Pontus and Aristarchus, who supposed that the phenomena can be saved if the heaven and the stars are at rest while the earth moves about the poles of the equinoctial circle from the west to the east, completing one revolution each day, approximately; the ‘approximately’ is added because of the daily motion of the sun to the extent of one degree”.

Heraclides made another important advance towards the Copernican hypothesis. He discovered the fact that Venus and Mercury revolve about the sun as centre. So much is certain; but a further question naturally arises. Having made Venus and Mercury revolve round the sun like satellites, did Heraclides proceed to draw the same inference with regard to the other, the superior, planets? The question is interesting because, had it been laid down that all the five planets alike revolve round the sun, the combination of this hypothesis with Heraclides’s assumption that the earth rotates about its own axis in twenty-four hours would have amounted to an anticipation of the system of Tycho Brahe, but with the improvement of the substitution of the daily rotation of the earth for the daily revolution of the whole system about the earth supposed at rest. Schiaparelli dealt with the question in two papers entitled I precursori di Copernico nell’ antichità (1873), and Origine del sistema planetario eliocentrico presso i Greci (1898). Schiaparelli tried to show that Heraclides did arrive at the conclusion that the superior planets as well as Mercury and Venus revolve round the sun; but most persons will probably agree that his argument is not convincing. The difficulties seem too great. The circles described by Mercury and Venus about the sun are relatively small circles and are entirely on one side of the earth. But when the possibility of, say, Mars revolving about the sun came to be considered, it would be at once obvious that the precise hypothesis adopted for Mercury and Venus would not apply. It would be seen that Mars is brightest when it occupies a position in the zodiac opposite to the sun; it must therefore be nearest to the earth at that time. Consequently the circle described by Mars, instead of being on one side of the earth, must comprehend the earth which is inside it. Whereas therefore the circles described by Mercury and Venus were what the Greeks called epicycles about a material centre, the sun (itself moving in a circle round the earth), what was wanted in the case of Mars (if the circle described by Mars was to have the sun for centre) was what the Greeks called an eccentric circle, with a centre which itself moves in a circle about the earth, and with a radius greater than that of the sun’s orbit. Though the same motion could have been produced by an epicycle, the epicycle would have had to have a mathematical point (not the material sun) as centre. But the idea of using non-material points as centres for epicycles was probably first thought of, at a later stage, by some of the great mathematicians such as Apollonius of Perga (about 265–190 B.C.).

Not only does Schiaparelli maintain that the complete (but improved) Tychonic hypothesis was put forward by Heraclides or at least in Heraclides’s time; he goes further and makes a still greater claim on behalf of Heraclides, namely, that it was he, and not Aristarchus of Samos, who first stated as a possibility the Copernican hypothesis. Now it was much to discover, as Heraclides did, that the earth rotates about its own axis and that Mercury and Venus revolve round the sun like satellites; and it seems a priori incredible that one man should not only have reached, and improved upon, the hypothesis of Tycho Brahe but should also have suggested the Copernican hypothesis. It is therefore necessary to examine briefly the evidence on which Schiaparelli relied. His argument rests entirely upon one passage, a sentence forming part of a quotation from a summary by Geminus of the Meteorologica of Posidonius, which Simplicius copied from Alexander Aphrodisiensis and embodied in his commentary on the Physics of Aristotle. The sentence in question, according to the reading of the MSS., is as follows: “Hence we actually find a certain person, Heraclides of Pontus, coming forward and saying that, even on the assumption that the earth moves in a certain way, while the sun is in a certain way at rest, the apparent irregularity with reference to the sun can be saved”. (The preceding sentence is about possible answers to the question, why do the sun, the moon and the planets appear to move irregularly? and says, “we may answer that, if we assume that their orbits are eccentric circles or that the stars describe an epicycle, their apparent irregularity will be saved, and it will be necessary to go further and examine in how many different ways it is possible for these phenomena to be brought about”.)

Now it is impossible that Geminus himself can have spoken of an astronomer of the distinction of Heraclides as “a certain Heraclides of Pontus”. Consequently there have been different attempts made to emend the reading of the MSS. All the emendations proposed are open to serious objections, and we are thrown back on the reading of the MSS. Now it “leaps to the eyes” that, if the name of Heraclides of Pontus is left out, everything is in order. “This is why one astronomer has actually suggested that, by assuming the earth to move in a certain way, and the sun to be in a certain way at rest, the apparent irregularity with reference to the sun will be saved.” This seems to be the solution of the puzzle suggested by the ordinary principles of textual criticism, and is so simple and natural that it will surely carry conviction to the minds of unbiassed persons. Geminus, in fact, mentioned no name but meant Aristarchus of Samos, and some scholiast, remembering that Heraclides had given a certain motion to the earth (namely, rotation about its axis), immediately thought of Heraclides and inserted his name in the margin, from which it afterwards crept into the text.

It is only necessary to add that Archimedes is not likely to have been wrong when he attributed the first suggestion of the Copernican hypothesis to Aristarchus of Samos in express terms; and this is confirmed by another positive statement by Aëtius, already quoted, that “Heraclides of Pontus and Ecphantus the Pythagorean made the earth move, not in the sense of translation, but with a movement of rotation”.